As the human genome is mapped and diseased-related genes identified, tests for screening populations for disease-causing or susceptibility- conferring mutations will become increasingly available. For some of the diseases, no effective cure will be available. Carrier screening will identify individuals and couples at risk of having affected offspring. They then can prepare for the birth of an affected child or choose options to avoid its conception or birth. Most people to whom carrier screening is offered will have little or no personal familiarity with the disease in question. Unless they receive sufficient information about the disease (extent of disability and range of severity), the limitations of the test (false positives and false negatives), and the implications of positive results (risk of having affected offspring, prenatal diagnosis, abortion) their decision on whether or not to be screened may not be consonant with their child- rearing goals, tolerance for uncertainty, or moral precepts. Among those who agree to be screened, the results may generate psychological stress and demand for additional testing even when the chance of having an affected child is relatively small. The availability of a test that detects most, but not all, carriers of the mutations that cause cystic fibrosis (CF) affords an opportunity to determine how people respond when they are confronted with the actual decision of whether to be screened. CF carrier screening will be offered to young adult members of a health maintenance organization. We will determine the rate of acceptance of screening following a group education program and examine the influence of demographic, socioeconomic, cognitive, and behavioral characteristics on the decision to be screened. The impact of screening on those identified as carriers, and on couples in whom one partner is identified as a carrier, will also be studied. The rates of acceptance of screening among a well-informed population and the effect of being identified as a carrier will provide better guidance to public policy related to genetic screening.